The present invention concerns an aqueous bath composition for electrolytic deposition of gold/silver alloys. Numerous such compositions are, of course, known in the art. It is also generally known in the art that certain additives may be included in a bath composition which have a brightening effect on the deposited electroplate metal.
For example, U.S. Pat. No. 3,864,222 issued Feb. 4, 1975, to William A. Wilson et al discloses the use of polyethyleneimines as an additive to gold and gold alloy plating baths as agents for general improvement of the brightness of the electroplated metal. The polyethyleneimines are added in the amounts of 0.005 to 100 grams per liter and have a molecular weight in the range 100 to 60,000.
U.S. Pat. No. 3,984,292 discloses a bath for the electrodeposition of silver from an aqueous cyanide-free bath including the use of organic nitrogen containing compounds, including polyethyleneimine, and a compound of sulfur or selenium in which the sulfur or selenium is in an oxidation state of -1 or -2.
It has now been found that in a gold/silver alloy low or no free cyanide plating bath, polyethyleneimine brightener tends to retard the electrodeposition of gold more than that of silver, with the result that employment of polyethyleneimine favors deposition of silver over gold and provides a lower karat deposit. It has further been found that satisfactory brightening of a gold/silver alloy is attained notwithstanding a reduction of the amount of polyethyleneimine employed when selenium in the -2 valance state is combined with the polyethyleneimine. It is accordingly an object of the present invention to provide a novel gold/silver alloy plating bath which enhances brightening of the electroplated metal alloy without adversely affecting the radio of gold to silver plated, and thereby the karat of the finished electroplate.
It is another object of the present invention to provide a novel bath composition for the electroplating of gold/silver alloys which is simple and stable under plating conditions and in which a synergistic brightening and karat stabilizing effect is attained by utilization of a brightening additive comprising a water soluble compound of selenium in the -2 valance state and a water soluble polyethyleneimine compound.
It is another object of the present invention to provide a novel bath composition for electroplating gold/silver alloys which provides a reduction of tensile stress in the electroplated alloy.